Skillful servicing and innovations such as mortgage insurance have helped reduce the risks associated with net interest margin securitizations, experts say. But the transactions – backed by residual cash flows – remain complex to rate. Prompted by its merger with Duff & Phelps, rating agency Fitch IBCA has released new criteria for evaluating net interest margin transactions. In general, the Fitch guidelines highlight the growing importance of prepayment penalties and loan servicing in the subprime